1. Field of the Invention
The present invention relates to a power supply unit and an image forming apparatus including the power supply unit.
2. Description of the Related Art
An image forming apparatus using an electrophotography process, such as copying machine, a printer, a facsimile machine, or a multifunction machine including these functions, includes a photosensitive element which is an image carrier, an image forming unit that includes a charging unit, an exposing unit, a developing unit, a transferring unit, and the like arranged around the photosensitive element, and a fixing unit (also referred to as a “fixing device”) for fixing a toner image transferred to a transfer sheet by the transferring unit. A fixing roller including a heater (fixing heater) is provided in the fixing unit, and a heater control device that controls supply of electric power to the fixing heater is also provided in order to maintain a surface temperature (fixing temperature) of the fixing roller to a target temperature.
In recent years, with multi-function of such an image forming apparatus, the structure thereof has also become complex and maximum power consumption tends to increase.
Further, the amount of power supplied (fed) to a fixing heater tends to increase in order to decrease a standby period of an operator and factors associated with an image forming apparatus itself such as a standby period before a fixing unit starts or a temporary halt of an operation due to a decrease in a fixing temperature during printing or copying.
On the other hand, since there is an upper limit power that can be supplied from a commercial power supply, when only a first power supply (main power supply) that supplies power from the commercial power supply is used, power consumption may exceed the maximum power consumption. Thus, an image forming apparatus that uses a second power supply (auxiliary power supply) that includes an electric storage device and a power supply circuit that uses electric power of the electric storage device as a power source and supplies electric power to a system by switching the first and second power supplies is already known.
However, in such an image forming apparatus, there is a possibility that an abnormality occurs in the output from the first power supply due to an overvoltage or an overcurrent, a voltage variation, and the like during a recovery (power failure recovery) from a power-cut state due to a power failure. In this case, there is a problem in that switching from the output (power supply) of the second power supply to the output of the first power supply is not properly performed, and a normal recovery is not realized.
Thus, for example, Japanese Patent Application Laid-open No. 2005-148581 discloses a technique in which in order to prevent deterioration of an image forming unit when a power failure occurs during printing, charged electric power of an auxiliary power supply (second power supply) used as a power supplying unit for supplying electric power to a fixing heater is supplied to an image forming unit to continue an image forming process during a power failure so that respective units are not damaged even when the supply of electric power from a main power supply is cut.
However, according to the disclosure of Japanese Patent Application Laid-open No. 2005-148581, the configuration of the second power supply is not clear, and a voltage variation when switching of the output of the power supply occurs during startup or operations of the system is not taken into consideration. Thus, it is difficult to solve the problem that when an abnormality occurs in the output of the first power supply during a power failure recovery, switching from the output of the second power supply to the output of the first power supply is not properly performed, and a recovery is not properly realized.
Therefore, there is a need to allow a recovery to be realized properly without causing an abnormality in the output of a power supply even when a power failure occurs.